In numerous industrial domains, the determination of the surface state or of the roughness of a piece, particularly made of metal, is of the highest importance. In fact, the roughness determines a large number of functions, in particular the conditions of friction between two pieces, their adherence, their surface energy, the adherence of an intermediate layer of glue serving to assemble two pieces, etc... It is therefore necessary, in these industrial applications, to determine with precision the roughness of the surface of the pieces in question. Such determination has up to the present time been effected by means of so-called "roughness-measuring" apparatus, which are generally of particularly complex structure and delicate to employ, or optical or interferometric, etc . . . methods.
As described in Patent FR-A-1 583 511, a device is known for recording the surface state of a piece which is constituted by a composite film comprising a thin layer of supple synthetic material, capable of undergoing temporary plastic deformations, superposed on a thin layer of malleable material capable of undergoing permanent plastic deformations, with the interposition between these two materials of a bonding product such as an adhesive. This composite film, once applied under pressure against a surface whose roughness must be characterized, enables a three-dimensional impression of this rough surface to be instantaneously made on the malleable material. Although such a device makes it possible easily to note the surface state of a piece, it does not allow an automatic and instantaneous interpretation of the deformation of the layer of malleable material characterizing the roughness of the surface.
It is an object of the present invention to overcome these drawbacks by providing a process and an apparatus of very simple design enabling precise, reliable measurements of the roughness to be very rapidly obtained.